1. Field of the Invention
This invention relates to PU-coating compositions containing solvent and combinations of crosslinking agents and to a process for producing polyurethane coatings on the principle of heat-activated, one-coat reverse coating by knife-coating the polyurethane coating composition onto a temporary support, drying the layer and laminating the textile substrate using preferably heated pressure rollers, followed by heating at elevated temperatures, the coating compositions used being solvent-containing, 20 to 60% compositions of a polyurethane mixture I containing
(A) relatively soft polyurethanes and PA1 (B) relatively hard one-component polyurethanes and a crosslinker combination II containing PA1 (C) formaldehyde-urea and/or formaldehyde-melamine resins and crosslinking catalysts and PA1 (D) aliphatic and/or aromatic polyisocyanates, particularly blocked polyisocyanates, PA1 (I) from about 60 to 97.5 parts of a polyurethane mixture of PA1 (II) from about 2.5 to 15 parts, preferably from about 4 to 10 parts of a crosslinker combination of PA1 (III)
and, optionally, additions of soluble cellulose acyl esters, handle-improving agents, optionally polysiloxanes containing oxyalkylene groups and other standard additives, such as pigments.
The heat-activated, one-coat reverse coatings obtained in this way show surprisingly high dry and wet adhesion values on textile substrates through the mixed crosslinking of the selected polyurethane mixtures and, hence, high article fastness values.
2. Description of the Prior Art
The production of artificial leather for various applications, such as outer clothing, shoe upper material or suitcase material, from polyurethane starting materials by the transfer process is already known. In general, the artificial leather is made up of two different layers, namely the surface layer and the adhesion layer. In general, from 20 to 50 g/m.sup.2 of solids are applied per layer. The starting materials used are solutions of one-component and also two-component polyurethanes or polyurethane dispersions. The two-component polyurethanes only receive their final plastic-like character through crosslinking with polyisocyanates.
A more recent technique for coating web-form textiles is the so-called heat-activated, one-coat reverse coating process. In this process, the coating paste is knife-coated onto a temporary support, for example a release paper, the solvent (mixture) is evaporated off, the textile substrate is laminated onto the dry, heat-activated polyurethane film using a heated pressure roll and the polyurethane film subsequently crosslinked at elevated temperature. The advantages of heat-activated, one-coat reverse coating lie in the low spread possible, for example from 15 to 30 g/m.sup.2, thereby enabling light coatings to be obtained. One particularly interesting aspect of this process is that it is possible to use substrates which are unsuitable or less suitable for solvent coating or for direct coating either on account of their constructions or on account of their dissolving properties (for example polyacrylate substrates or so-called PU-coagulates). Particularly light or open-mesh fabrics may also be coated without any danger of lamination failures.
Disadvantages of heat-activated, one-coat reverse coating process center above all around the adhesion problems experienced with articles produced by that process. The adhesion values vary and the wet adhesion values in particular are relatively poor.
Accordingly, the object of the present invention was to provide a composition for the heat-activated, one-coat reverse coating process which shows improved properties and, more particularly, improved adhesion of the articles produced by that process. According to the invention, this object is achieved by reacting a special polyurethane composition with a combination of formaldehyde crosslinkers and polyisocyanate crosslinkers, preferably blocked polyisocyanates.